Generally, such devices on double chainstitch sewing machines serve the purpose of controlling the feed and removal of the looper thread to the looper during formation of the stitch. The spreader has the task of diverting to the side or pulling out the thread which protrudes out of the looper point and runs upwards to the stitch hole so that the descending point of the needle can move past between said thread and the looper. At this moment the looper thread should be slightly tensioned so that the so-called insertion movement, i.e. the insertion of the point of the needle into the thread triangle, two sides of which are formed by the needle thread and the other side by the looper thread, is accurately achieved.
Machines of this type are often equipped with a feeding device for the material to be sewn, the transport taking place at the same time as the needle penetrates the material. Such feeding devices are, for example, described in the German Offenlegungsschrift No. P34 11 217.0 corresponding to U.S. patent application Ser. No. 06/702,038. With such so-called needle-transport sewing machines the needle adopts different positions for different stitch lengths when it enters the thread triangle formed with the aid of the spreader, i.e. during the insertion movement, such a measure reducing the insertion accuracy especially with larger stitch lengths of 6 and more millimetres. After the insertion operation the spreader must not hinder the looper thread in any way when the latter is withdrawn from the spreader through the looper movement.
The procedure described above results in certain demands being placed on the sequence of movements of the spreader while it is acting on the looper thread. The remaining sequence of movements of the spreader, which lies inactive immediately adjacent to the looper, must be designed in such a way that a collision with the looper is absolutely precluded.
Furthermore, it is also desirable for the short-time action of the spreader on the under thread to be achieved by a spreader movement which exhibits a small amplitude and which thus permits another looper to be arranged at the smallest possible distance from the first looper so that parallel stitches can also be made.
A device of the generic type is known from a publication of Pfaff, Kaiserslautern, Federal Republic of Germany, "PFAFF adjusting instructions Pfaff 5642, No. 296-12-13 924". According to this publication the spreader is driven by a link mechanism which only satisfies the requirements placed on the movements of the spreader to a limited extent. In order to produce stitches with a maximum stitch length of 6 mm, spreaders whose points are bent are provided. These measures have an influence on the release of the looper thread and serve to compensate for inadequacies in the sequence of movements of the spreader. In addition to these spreaders, which are to be adjusted with great care, this device is also designed to control the looper thread with thread eyes in the thread delivery zone, said thread eyes adopting different positions in relation to each other depending on the set stitch length. These thread eyes can be adjusted for work with threads of different elasticity. The device is not suitable for influencing stitch formation to such an extent as to achieve an accurate stitch formation and good stitch tightening for stitch lengths which are considerably longer than 6 mm as well.